Vapor-liquid contacting devices, such as fractionation trays and packings, are employed to perform an almost endless variety of separations in the petroleum and petrochemical industries. Fractionation trays are used in the separation of many different hydrocarbons including paraffins, aromatics and olefins. Fractionation trays are also used to separate specific compounds such as alcohols, ethers, alkylaromatics, monomers, solvents, inorganic compounds, and atmospheric gases in the separation of broad boiling mixtures such as petroleum-derived fractions. Vapor-liquid contacting trays are also used to perform gas processing, purification, and absorption.
While a wide variety of fractionation trays and other contacting devices with differing advantages and disadvantages have been developed, fractionation trays and packings are the predominant form of conventional fractional distillation apparatus. They are widely used in the chemical, petrochemical and petroleum refining industries to promote vapor-liquid contacting performed in vessels such as fractionation columns. Trays are mounted horizontally, typically at uniform vertical distances referred to as the tray spacing of the column.
Fractional distillation has traditionally been conducted in cross flow or counter current contacting devices that exhibit an overall downward liquid flow and upward vapor flow. The vapor and liquid phases are brought into contact at contacting stages to allow the vapor and liquid phases to exchange components and approach equilibrium with each other. The vapor and liquid are then separated, moved in the appropriate direction and contacted again with another quantity of the appropriate fluid. In many conventional vapor-liquid contacting devices, vapor and liquid are contacted in a cross flow arrangement at each stage. An alternative apparatus differs from traditional multi-stage contacting systems in that while the overall flow in the apparatus continues to be countercurrent, each stage of actual contacting between the liquid and vapor phases is performed in a co-current mass transfer zone.
During the fractional distillation process using co-current vapor-liquid contacting devices, vapor generated at the bottom of the column rises, coming into contact with liquid exiting downcomer. The contact between the vapor and liquid causes the liquid to form droplets which are entrained in the vapor and carried upward. As a result, the vapor and liquid share a flow path for at least a short period. During the period of entrainment, a compositional equilibrium between the vapor and liquid phases is approached. During mass transfer, the vapor loses less volatile material to the liquid and thus becomes slightly more volatile as it passes upward through the vessel. Simultaneously the concentration of less volatile compounds in the liquid increases as the liquid moves downward from tray to tray. The liquid laden vapor passes through a separation device where the liquid is removed. The liquid separated from the vapor travels downward to the next lower tray. This continuous entrainment and vapor-liquid separation is performed on each tray. Vapor-liquid contactors therefore perform the two functions of contacting the rising vapor with liquid and then allowing the two phases to separate and flow in different directions. When the steps are performed a suitable number of times, the process leads to separation of chemical compounds based upon their relative volatility.
Two very important characteristics of vapor-liquid contacting equipment in which improvement is always sought are capacity and efficiency. A co-current contacting device is believed to be one apparatus for achieving high capacity through use of vapor-liquid separation devices such as demisters or centrifugal vanes that enhance vapor-liquid separation at each stage. The co-current contacting device can also achieve high mass transfer efficiency through the co-current contacting of fine liquid droplets with vapor.
If an unbalanced or maldistributed liquid flow occurs in a vapor-liquid contacting apparatus from operation under non-vertical conditions, i.e., if the vessel itself is rocking or tilted, fluid may not be readily redistributed along the mass transfer zones of the apparatus. The maldistribution of liquid may propagate from one stage to the next, reducing the capacity and efficiency of the apparatus.
Accordingly, it is desirable to provide a co-current vapor-liquid contacting apparatus that inhibits or eliminates unbalanced or maldistributed liquid in contacting stages. It is also desirable to provide a co-current vapor-liquid contacting apparatus that inhibits liquid flow horizontally within and between contacting stages under non-vertical conditions. Furthermore, other desirable features and characteristics of the co-current vapor-liquid contacting apparatus will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.